The present invention relates to an optical fiber connecting device for connecting optical fibers and, more particularly, to a structure of such a device that is simple yet capable of achieving great reduction in the insertion loss incurred in connectors connecting single-mode optical fibers used in long-distance large-capacity communications.
Optical fiber cores which are used in single-mode optical fibers to transmit optical signals have, in general, a diameter of 7 to 10 .mu.m in order to reduce transmission loss. The insertion loss incurred by connecting optical fibers by means of optical connectors is influenced by various factors. It is known that the greatest of these factors is the deviation of the axial centers of optical fiber cores which oppose each other and are connected to each other. It is said that if, for instance, the diameter of an optical fiber core is 10 .mu.m, a deviation of 1 .mu.m of the axial center causes an insertion loss of about 0.25 dB; similarly, an axial-center deviation of 2 .mu.m causes an insertion loss of about 0.75 dB, and an axial-center deviation of 3 .mu.m causes an insertion loss of about 1.8 dB. It is desired that the insertion loss should be within a range in which the maximum is 0.5 dB and which averages 0.2 dB. In order that the insertion loss be within this range, the deviation of the axial center of an optical fiber core must be within a range in which the maximum is 1.6 .mu.m and which averages 0.9 .mu.m. Actually, however, the deviation of the axial center of an optical fiber core relative to the center of the outer diameter of the ferrule is influenced by other factors, such as errors incurred during the production of the optical fibers per se and the ferrules, as well as fitting errors incurred during the assembly of the optical fibers onto the ferrules. As a result, the axial-center deviation amounts to about 3 to 4 .mu.m at most and averages about 1.5 .mu.m, making it impossible, at present, to achieve an insertion loss within the desired range.
Hitherto, various methods have been proposed based on the fact that, in most of the optical fiber connectors combined with optical fibers, the cumulative influence of inevitable errors such as those described above causes a phenomenon in which the axial center of each optical fiber core is more or less eccentric with respect to the outer-diameter center of the ferrule. These methods are adapted to equalize the angles at the maximum eccentric positions of the axial centers of the optical fiber cores with respect to the outer-diameter centers of the ferrules of a pair of optical connectors, thereby minimizing the axial-center deviations of the optical fiber cores. However, these methods are not effective enough because they encounter technical difficulties concerning the production of optical connectors or because they are limited to adjustment in several stages.